Biology Forums - Study Force

NASA has discovered an alien life-form. You are called in to help NASA scientists to deduce the genetic code for this alien.
 
  Surprisingly, this alien life-form shares many similarities with life on Earth: this alien uses DNA as its genetic material, makes RNA from DNA, and reads the information from RNA to make protein using ribosomes and tRNAs. Even more amazing, this alien uses the same 20 amino acids, like the organisms found on Earth, and also codes for each amino acid by a triplet codon. However, the scientists at NASA have found that the genetic code used by the alien life-form differs from that used by life on Earth. NASA scientists drew this conclusion after creating a cell-free protein-synthesis system from alien cells and adding an mRNA made entirely of uracil (poly U). They found that poly U directs the synthesis of a peptide containing only glycine. NASA scientists have synthesized a poly AU mRNA and observe that it codes for a polypeptide of alternating serine and proline amino acids. From these experiments, can you determine which codons code for serine and proline? Explain.
 
  Bonus question. Can you propose a mechanism for how the alien's physiology is altered so that it uses a different genetic code from life on Earth, despite all the similarities?

No, you cannot definitively determine the codons that code for serine or proline, because it could be either UAU or AUA.

Bonus. The alien aminoacyl-tRNA synthetases could adapt a different amino acid to each tRNA, thus matching an amino acid with a different codon compared with the codons used by life on Earth.